Not applicable.
1. Field of the Invention
The subject invention relates to a method for determining the oil and grease content in an aqueous solution. More specifically, the subject invention relates to a method for measuring the oil and grease content in an aqueous solution on offshore oil and gas facilities, where persistent wave motion, vibration, and/or other environmental and/or physical conditions make conventional methods of measurement inadequate.
2. Description of Related Art
The U.S. Environmental Protection Agency (xe2x80x9cEPAxe2x80x9d) and various state governments have enacted regulations and statutes to control the amount of oil and grease being discharged from oil and gas facilities engaged in operations such as production, field exploration, developmental drilling, well completion, and well treatment. For instance, the EPA authorizes operators engaged in such oil and gas operations off the coast of Louisiana to discharge xe2x80x9cproduced waterxe2x80x9d into the territorial seas of Louisiana, but only if the oil and grease concentration of the produced water has a daily maximum of 42 ppm and a monthly average of 29 ppm. See Final NPDES General Permit for Discharge from new and Existing Sources in the Offshore Subcategory of the Oil and Gas Extraction Category for the Territorial Seas of Louisiana (LAG260000). Other governmental agencies such as the Minerals Management Service may govern produced water discharge in Federal waters.
xe2x80x9cProduced waterxe2x80x9d is partially derived from a natural water layer (called formation water) that typically lies underneath the hydrocarbons in reservoirs of oil and gas (although at times the water may be effectively intermingled with the hydrocarbons). To achieve maximum hydrocarbon recovery from these reservoirs, additional water is occasionally injected into the reservoirs to help force the hydrocarbons to the surface. Both the formation water and the injected water may be eventually produced along with the hydrocarbons. Hence, the name xe2x80x9cproduced waterxe2x80x9d applies to the formation water and injected water that is sometimes produced from oil and gas reservoirs.
At the surface, the produced water is separated from the hydrocarbons, and thereafter treated to remove as much oil and grease as possible. Even after treatment, however, the produced water still contains traces of oil and grease, which presents an environmental disposal problem that is closely monitored by the EPA and various federal and state entities, as discussed above. Thus, for the purpose of regulatory compliance and process control, measurement of oil and grease in produced water has been and will remain a very important aspect of the oil and gas industry.
The EPA recommends that laboratories use a liquid-liquid extraction procedure for determining the oil and grease content in produced water. During a typical liquid-liquid extraction procedure, a solvent is added to a known volume of produced water containing some amount of oil and grease. The solvent extracts the oil and grease, and the solvent is thereafter separated from the produced water. Next, the solvent is evaporated until all that remains is the extracted oil and grease. The weight of the extracted oil and grease is then compared to the volume of the produced water to determine the oil and grease concentration.
For oil and gas operations, especially those operations performed offshore, where constant wave motion, vibration, and/or other environmental and/or physical conditions are prevalent, obtaining an accurate weight of the extracted oil and grease during the final stages of the liquid-liquid extraction procedure has proven to be very difficult. Conventional weighing devices typically used for this application are incapable of providing accurate measurements unless they are level and stationary, two conditions which infrequently occur on offshore facilities.
To avoid the effects of constant wave motion, vibration, and/or other environmental and/or physical conditions, offshore operators currently ship their produced water samples to shore-based laboratories for oil and grease analysis. The shipment of these samples to shore-based laboratories, either by boat or helicopter, is time consuming and costly. Shipping by boat is relatively slow and limited by rough sea conditions. Shipping by helicopter is very expensive and limited by inclement weather and nighttime conditions. Delays associated with the shipment of samples can result in forced production downtime with the attendant loss of revenue. It is readily seen, therefore, that accurate, reliable oil and grease measurement has tremendous economic incentives.
The present invention provides a method for determining the oil and grease content of an aqueous stream generated from an offshore facility, with the measurement being conducted on the offshore facility without having to ship samples of the produced water to shore-based laboratories for analysis. The present invention discloses a method by which samples can be analyzed offshore, regardless of the wave motion, vibration, and/or other environmental and/or physical conditions that are prevalent offshore. The present invention therefore avoids the potential time delays and costs associated with the shipment of samples from offshore operations to shore-based laboratories.
The related prior art references include U.S. Pat. No. 5,294,553, issued to Kawahara on Mar. 15, 1994, which discloses a method of gravimetric determination of grease and oil in an aqueous matrix, comprising the steps of preparing a sample, extracting the grease and oil using a non-halogenated solvent mixture, distilling and evaporating the solvent, and weighing the extraction residue.
Another related prior art reference is U.S. Pat. No. 5,178,227, issued to Kvisgaard on Jan. 12, 1993, which discloses a maritime weighing system that makes corrections for errors due to tiltings of the ship and due to the mechanical noise produced by the engine of the ship or by operative implements on the ship.
None of these prior art references, nor any of the prior art references listed in the Applicant""s Information Disclosure Statement, disclose the method of the present invention.
The present invention provides a method for determining the oil and grease concentration of aqueous solutions on an offshore facility without having to ship the aqueous solutions to shore-based laboratories for analysis. The method of the present invention generally involves a liquid-liquid extraction procedure, where the weight of oil and grease extracted from a known volume of aqueous solution is determined using a pair of weighing devices, which are connected to a common computer that is capable of processing synchronized data points received from the two weighing devices.
The first weighing device is a control scale upon which a known weight is placed. The second weighing device is a sample scale upon which the extracted oil and grease is placed. Any wave motion, vibration, and/or other environmental and/or physical condition on the offshore facility affecting the first weighing device will also affect the second weighing device in like fashion, and therefore will be compensated for when determining the true weight of the oil and grease. The present method allows the aqueous solution samples to be analyzed offshore and, therefore, eliminates the time delays and costs associated with the shipment of samples from offshore facilities to shore-based laboratories.